This is a list of various possibilities that could be true, and which would be very frightening if they are. They are varied in scale an effect, but are all unpleasant. I have left out nuclear war and natural disasters (such as super volcanoes) as we know they are possible. I am interested in things that have a reasonable likelihood of being try, but are not known.
Industrial/technological civilisation is inevitably doomed
Lets start with the most grandiose and pessimistic ideas. There are, in fact, several possibilities here, united in that they are explanations of the Fermi paradox, which means that they must apply to any civilisation (not just ours) and any intelligent beings (not just humans). From the Drake equation there should be someone out there, but we cannot find them. An inevitable doom is one explanation.
So, we what could this inevitable doom be?
- The inevitable invention of a technology that destroys civilisation. We have already invented nuclear weapon and biological weapons, and are likely to invent more. There is a fair amount of potential for biotechnology or nano-technology to produce something nasty enough to bring down civilisation.
- Resource shortages: his hardly needs explanation. We know we will run out of oil and other fossil fuels, as well as some metals (lithium for example). Even basic materials like wood are in danger. All of these are needed to sustain out civilisation. So far we have kept ahead of Malthus with advancing technology, but there may be some limit that will end this and stabilise the world at a minimal standard of living. See below for more.
- Natural disaster: not the most probable. We know that really large natural disasters (such as those that caused mass extinctions) happen tens of millions of years apart. This is too infrequent, and, unless the earth is a very safe place compared to other planets the maths will not work.
- The berserker explanation. This is named after Fred Saberhagen’s berserker books. There is a hostile force (or forces) that destroy every civilisation they find. They may do this to ensure potential rivals never do the same to them, or as a result of past wars. All civilisations are either destroyed or hide themselves, maintaining radio silence.
As we do not maintain radio silence, and, at the very least, anyone with a radio telescope within a 60 light year or so (and expanding) radius will see that there is something in the solar system emitting far more microwave and radio radiation over many wavelengths than the sun should. Given how useful these are (we use them for cooking, radar, communications, medicine and more) it is hard to imagine any technological civilisation not emitting a comparable amount.
One of the commonest criticisms of global warming (as a hypothesis) is that until recently scientists expected a global cooling leading to a new ice age — so why should we think the models are right this time? Suppose that the models were right both times. How can this be possible? It could be true if we were heading for an ice age that was warded off by anthropogenic global warming.
We unlikely to be able to control human activity to balance the two because we would need very accurate models and iron discipline to do so. It would also be made impossible by any positive feedback effects (e.g. from growing/shrinking ice sheers reflecting more/less sunlight back into space).
If we are lucky we may have a choice about whether to regulate human activity to bring about a hothouse or an ice-age.
Some one nasty has some of the missing uranium
A lot of uranium (and other dangerous substances) seems to go missing, much of it weapons grade. Not only is Uranium missing in Iran, but over the years almost every country that has fissile materials seems to have lost some, including some quite large amounts in the US. States that mine uranium include some pretty unstable places, and one wonders whether it really all tracked from mine onwards in the first place — aside from what we know they have lost.. The hundreds of failed attempts to smuggle fissile materials out of Russia alone suggest that some must have been successful.
On the positive side, a small nuclear bomb will only get a section of a reasonably large city, so we are not talking about an end of the world scenario here. On the other hand most press coverage only looks at weapons grade uranium, whereas what is used in civilian reactors (of which there are many) is quite adequate to make a dirty bomb.
Big solar flares are more likely than we think
The big shiny thing in the sky is constantly flaring up like a teenager and causing interference with things like short-wave radio. Most of the time its nothing most of us would even notice. However, a really big solar flare would cause a lot of damage, knocking out telecommunications, satellites and even electricity grids. A big solar flare in 1859 not only set telegraph systems on fire, it generated enough electricity to allow them to operate with power disconnected. The effect on more delicate modern electronics could be much worse. In a society dependent on computers and telecommunications a really big solar flare, or series of them, could d enough damage to bring essential services to a stand still: maybe even for long enough to bring down civilisation. A few days of disruption to power, food and communications could suffice.
We have reached the limits of technological advance
One possible explanation for the slowdown in technological innovation is that we are reaching the limits of what is possible. All the basic key technologies that matter have been invented, and there is no more to come. As continuing technological advance is what had kept us ahead of Malthus, providing replacements for scarce materials, the ability to grow and transport more food, and allowing us to use resources more efficiently, the result of a failure to keep advancing will be reducing resources per capita, and falling living standards: until we reach the point where the death rate (from malnutrition and disease) rises to stabilise the population.